In recent years, attention has been focused on small storage systems tailored to high energy density applications including information-related equipment and communication equipment such as personal computers, video cameras, digital still cameras and mobile phones and on large ones tailored to power applications including auxiliary power supplies for electric vehicles, hybrid vehicles and fuel cell vehicles and electric power storages. As a candidate, a nonaqueous electrolyte battery such as a lithium-ion battery, a lithium battery and a lithium ion capacitor has been diligently developed.
Most of these nonaqueous electrolyte batteries have already gone into actual use; however, these are not satisfying in terms of durability in the various uses, for example, in a long period of use under high temperatures or in the use for automotive vehicle, since these are remarkably deteriorated particularly at temperatures of not lower than 45° C.
In general, a nonaqueous electrolyte solution or that quasi-solidified with a gelling agent is used as an ion conductor in these nonaqueous electrolyte batteries, and is arranged in such a manner that aprotic one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like and any combination thereof is used as a solvent and that a lithium salt such as LiPF6, LiBF4, (CF3SO2)2NLi and (C2F5SO2)2NLi is used as a solute.
As a means of improving the nonaqueous electrolyte batteries in durability e.g. cycle characteristics and high-temperature storage characteristics, it has hitherto been considered to optimize various battery components such as the positive or negative active material. Nonaqueous electrolyte solution-related technologies are no different therefrom, in which it is proposed to inhibit, with various additives, the electrolyte solution from decomposing on an active positive or negative electrode surface and therefore from deteriorating. For example, a proposition made by Japanese Patent Provisional Publication No. 2000-123867 is to improve the electrolyte solution in battery characteristics with the addition of vinylene carbonate. This method is for preventing the electrolyte solution from decomposing on the electrode surface by coating the electrode with a polymer film formed by polymerization of vinylene carbonate, whose problem is that lithium ions also have difficulty in passing through the film so that the internal resistance is increased.
In Japanese Patent Publication No. 3439085, it is disclosed that high-temperature cycle characteristics are improved by the effect of the film formed on the electrode's interface by adding lithium difluorophosphate to the electrolyte solution. However, the synthesis of lithium difluorophosphate thus used as an additive is so difficult that an effective production method applicable to industrial production has not been established. Japanese Patent Provisional Publication No. 2005-219994 discloses that lithium difluorophosphate is produced by reacting lithium hexafluorophosphate with silicon dioxide at a reaction temperature of 50° C.; however, it requires a very long time until the end of the reaction, and more specifically, three days. Though a method of increasing the reaction temperature is conceivable in order to improve the reaction rate, lithium hexafluorophosphate begins to decompose when the reaction temperature exceeds 60° C. to cause deterioration of the electrolyte solution, which is also problematic. Further, in a method for producing lithium difluorophosphate by reacting hexafluorophosphate with water (Non-Patent Document 1), pure lithium difluorophosphate has not been obtained since acids such as hydrogen fluoride, difluorophosphoric acid and monofluorophosphoric acid are generated and additionally such acids are hard to be removed. Moreover, lithium difluorophosphate isolated from the solution is unstable among difluorophosphates and is acceleratingly decomposed due to the coexistence of the above-mentioned acids, so as to have never been actually isolated.
Patent Document 1: Japanese Patent Provisional Publication No. 2000-123867
Patent Document 2: Japanese Patent Publication No. 3439085
Patent Document 3: Japanese Patent Provisional Publication No. 2005-219994
Non-Patent Document 1: J. Fluorine Chem. 126 (2005) 27